Cleaning and disinfection of potable water tanks

The situation where there is definitely the greatest risk of running out of potable water on board an offshore vessel is when the potable water tanks are to be maintained. There are usually either two or three drinking water tanks on board, and you end up in an extremely vulnerable situation when you take one of them out of service. By having a good plan one can reduce the risk of water shortages and often demobilization and shutdown of production considerably.


The storage tanks


How many people live on board? How much water do we have in reserve? How long do we manage with the water reserve before we run out of water? It is important to remember that showers, washing machines, dishwashers and toilets use a lot of water. Many people think that “we are fine without drinking water, we have bottled water on board!”. How long do you think it will take without water for the toilets before conditions on board become quite civilized? How is access to water? Can one produce its own fresh water from seawater? Is it possible to bunker water from the supply vessel?


In some cases, it may be advisable to disinfect the potable water tanks at down to 5 mg / l, so that one can use the water as potable water should something unexpected happen during the maintenance of the tanks, or with access to water. Water production units such as evaporator or reverse osmosis may fail. Technical problems onboard supply boat or bad weather can make it impossible to receive clean water from a boat.


It is critical that only clean equipment and protective equipment is used during maintenance in the potable water system. There have been several instances where forgotten equipment such as filler, t-shirts and fluorescent luminaire (!) Has caused major problems with the potable water quality after tank cleaning. The high pressure washer used should only be used for the potable water tanks. This also applies to any submersible pumps or suction hoses. It is a very bad idea to use the same bilge pump that was just used to suck out the last remnants from the septic tank.


One should also be aware that any paint coatings in the tanks must be improved. Under good conditions, the tank will then be out of operation for about a week. If you fill up the tank before the paint is properly cured, perhaps because you miscalculate the curing time due to too cold steel, you can risk that the tank is out of service for a long time. In some cases, the water quality will not be good until the tank is sandblasted and repainted.

Inspection and cleaning

When the plan is ready and everything is ready, you start by completely emptying the tank of water. Everything has to go out. If there are no drainage options, the water must be pumped out.


When the tank is empty, it must be inspected. The first thing we see then is how dirty it is. How much sludge there is on the bottom of the tank and the interior surfaces. Based on the condition of the tank, it should be considered whether the tank is cleaned often enough. If there is very much sediment bottom and coating on all surfaces it is clear that the tank should have been cleaned before. Ideally, the tank should be cleaned before it becomes hygienic.


Tip: Remember to take lots of pictures!


Once the condition of the tank is well documented, cleaning can begin. Of course, water with drinking water quality must be used to clean the tank. The service water, technical water or the like of the device which may provide the high pressure washing system may be contaminated by both bacteria and hydrocarbons. The bacteria will be removed during the subsequent disinfection of the tank while the hydrocarbons will remain. Drinking water system oil can create problems for a long time and it is often very difficult to trace the origin of the problem afterwards.


The surfaces are flushed with high pressure. Sometimes it will be necessary to use cleaning chemicals.

“It is no longer required for soaps to be approved by the Food Safety Authority, but there is a requirement that there should be nothing dangerous in the water when the cleaning is complete. This basically means that you can wash with whatever you want, as long as one can document that there are no harmful substances in the water when the cleaning is done. “

Biofilm stained white by incompletely cured zinc based paint.

Precipitation in a potable water tank that only receives water bunkered from land. This water has a higher content of organic compounds compared to water produced from seawater.

It is also often necessary to scrub the tanks with stiff brushes. All washing water and soap must of course be removed from the tank.


After cleaning, the tank should be inspected to determine if the cleaning has been thorough enough. Don’t be afraid to speak up if it’s not good enough! Take lots of pictures to document that the tank is free of ALL precipitation and any biofilm.


The condition of the tank protective coating is also assessed. Pay special attention to pit corrosion.

Note: For extensive rust attacks where more than 3% of the surface coating has rust impacts and the rust is scattered (is not located in a few places in the tank), the coating should be completely renewed.


Renewal should also take place if the coating is no longer smooth and hard, if it is difficult to clean (poor coating can give rise to biofilm, and cause odors and taste problems). Even properly applied coatings will normally need to be renewed approximately every 15 years, unless it can be documented that existing coatings are still of good quality. In this case, the date in the maintenance system must be set for when the next quality assessment of the coating is to take place.

Illustration showing how much 3% corrosion of a surface actually is.


Pit corrosion is a form of extremely local corrosion that leads to the formation of small holes in the metal. The driving force for pit corrosion is the passing of a small area, which becomes anodic, while an unknown but potentially large area becomes cathodic, leading to very local galvanic corrosion. The corrosion penetrates into the mass of the metal, with a limited diffusion of ions.


When the condition of the tank, whether hygienic or corrosion protection is satisfactory, it must be disinfected. This can be done in various ways but the most predictable and stable method is to fill the entire tank with water added hypochlorite while circulating the contents of the tank through pumps and back of the tank. This ensures good and uniform mixing of hypochlorite in the body of water and prevents the hypochlorite from sinking to the bottom. Hypochlorite is heavier than water and will sink to the bottom if the water stagnates.


When the tank is full of water and disinfectant hypochlorite, the concentration of free chlorine should be measured. Use a chlorine calculator to calculate how much chlorine you need to add to the tank. Initially, the tank should be disinfected with 10 mg / l of free chlorine for 12 hours.


Chlorine calculator


There are simple and handheld meters. It may be necessary to dilute the sample as several instruments have an upper range of 8 mg / l of free chlorine. One then takes a mixture of 1/2 pure water (bottled water can be used if it does not have too much minerals) and 1/2 sample from the tank. The result you get after following the instrument’s procedure is then multiplied by two to give us the concentration in the tank. The measurement uncertainty of the analysis is doubled at such a dilution (and further increased at greater dilution), but it is often the only way to measure higher values.


Log the result. The tank should now be isolated for 12 hours, preferably with the most turbulence on the water.


After 12 hours, the concentration of free chlorine is measured again. A possible decrease in concentration is due to the reactive chlorine compounds having reacted with organic compounds from the tank’s (inadequately cleaned?) Surfaces or from the water used for disinfection. If the drop in chlorine concentration is higher than 20%, the tank should be disinfected again using the same procedure. The water must then be dumped before any further attempt.